EXAMINATION - (Task Description)
|| When an audio tape becomes suspected
of tampering, it may be forwarded to a qualified forensic audio
specialist for authentication. Examples of such problems are:
Credibility questions relating to the tape recorder operator,
chain-of-custody contradictions, and differences between the content
of the tape and testimonies of what was said.
Most often a forensic expert is contacted when the tape is believed
to have been altered or tampered with. Due to the nature of the allegations
surrounding tampering issues, the examiner required specific items from
the patron. The Federal Bureau of Investigation, for example, has a list
of required information including:
|| The original tape.
|| The tape recorders and related components used
to produce the recording.
|| Written records of any damage or
maintenance done to the recorders, accessories, and other submitted
A detailed statement from the person or persons who made the recording,
describing exactly how it was produced and the conditions that existed
at the time, such as:
|| Power source, such as portable generator or
|| Input, such as telephone, radio frequency transmitter/receiver,
miniature microphone, etc.
|| Environment, such as telephone transmission
line, restaurant, apartment, etc.
|| Background noises, such as television, radio,
unrelated conversations, computer games, etc.
|| Foreground information, such as
number of individuals involved in the conversation, general topics
of discussion, closeness to microphone, etc.
|| Magnetic tape, such as brand, format, when purchased,
whether previously used.
|| Recorder operation, such as number
of times turned on and off in the record mode, type of keyboard or
remote operations for all known events, use of voice-activated features,
|| A typed transcript of the entire
recording or, if that is not available, transcriptions of the portions
The items listed above are examples of what is required by a forensic
expert as he begins an examination of questioned audio recordings.
Falsification of Tapes
A qualified forensic expert determines authentication by performing a
number of scientific tests which detect evidence of tampering or falsification.
Four basic types of tampering include:
Deletion - the elimination of words or sounds by
stopping the tape and over-recording unwanted areas.
Obscuration - the mixing in of sound of amplitude
sufficient to mask wave form patterns which originally would show stops
and starts in inappropriate places.
Transformation - the rearranging of words to change
content or context.
Synthesis - the adding of words or sounds by artificial
means or impersonation.
Electromechanical Indications of Such Falsification (Anomalies)
Gaps - segments in a recording which represent
unexplained changes in content or context. A gap can contain buzzing,
humming, or silence.
Transients - short, abrupt sounds exemplified by
clicks, pops, etc. Transients may indicate tape splicing.
Fades - gradual loss of volume. Fades can cause
inaudibility and are considered gaps when the recording becomes fully
Equipment Sounds - inconsistencies of context caused
by the recording equipment itself. Common equipment sounds include hums,
static, whistles, and varying pitches.
Extraneous Voices - background voices which at
times appear to be near as the primary voices. These can at times even
block the primary voices.
Methods FTA Uses to Detect Falsifications and Authenticate
A forensic expert is trained to correlate his observations of such anomalies
with machine functions to interpret events in the following ways:
Critical Listening: Use of human analytical capabilities
to locate anomalies. The forensic scientist listens with proper headphones
to the original tape using high quality analytical equipment. He first
performs a preliminary overview of the original tape and notes recorded
events including starts, stops, speed fluctuations, and other variations
requiring further investigation. He then examines record events and categorizes
them as environmental or non-environmental. After examining any anomalies,
the expert analyzes background sounds. He listens for abnormal changes,
absences, or differences in environmental sounds. The final phase of critical
listening is an extensive audit of the foreground information. He concentrates
on voices, conversation and other audible sounds. Here anomalies include
sudden changes in a person's voice, abrupt unexplained topic change, or
strong foreground interruptions indicative of obscuration. The initial
forensic process of critical listening provides foundation and direction
for later intensive tests.
Physical Inspection: The forensic expert next inspects
for tampering with a thorough visual inspection of the tape itself. He
inspects the housing for pry marks, welding, size, label and date consistent
with alleged recording date. He also measures the tape and assures the
splicing of magnetic tape to the leader is consistent with normal manufacturing
process. Any other splices are noted as possible alteration.
Magnetic Development: Direct visual observation
of the magnetically "developed" tape is conducted to find track
widths, the type of recorder used, and the presence or absence of residual
Spectrum Analysis: Specialized computer equipment
and programs to produce a visual interpretation of a frequency-versus-amplitude
and frequency-versus-amplitude-versus-time displays. This allows the expert
to view the entire spectrum or to zoom in on an area of particular interest
thereby helping to characterize the acoustic quality of anomalies and
identify their source.
Waveform Analysis: A computer generated display
representing time-versus-amplitude of recorded sounds in graphic form.
With such analysis the expert can sometimes measure signal return time
which reveals how long a recorder had been turned off. He can identify
record-mode including the measurement of record-to-erase-head distances,
determination of the spacing between gaps in multiple-gap erase heads,
and inspection of the signature shape and spacing of various record event
Recorder performance: various electrical and mechanical
measurements of standard and modified recorders for use in finding possible
origins of buzz sounds, hum, etc.
In order to submit sound recordings as evidence in court proceedings,
an attorney must prove that the tape is an authentic representation of
the conversation it is said to record. The traditional method of establishing
authenticity involves maintaining a chain of custody which logs all persons,
times and locations concerned in the creation of the tape. However, even
if this procedure is strictly observed, there may still be a challenge
to the tape's authenticity.
The recording may contain inconsistencies suggestive of tampering. In
such cases, an attorney may consult a qualified forensic examiner to inspect
the tape. The examiner would initially listen critically for signs such
as gaps, transients, fades, equipment sounds or extraneous voices which
indicate tampering. Then he would utilize other methods like physical
inspection, magnetic development, spectrum analysis, and waveform analysis
to discover anomalies. It is relatively easy to change the content of
a recording by deleting words or sections; by obscuring meaning with over-recorded
sounds; or by transforming context through rearrangement of selected phrases
or by adding additional words through synthesis. Nevertheless, falsifications
normally leave detectable magnetic and waveform acoustic signatures which
can lead to forensic individualization of the evidential recorders and